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DP83TC811S-Q1: SGMII AC capacitor placement

Arie Li
Expert 3390 points
Part Number: DP83TC811S-Q1

Hi experts,

Just want to double check with you, where should we place the SGMII AC capacitor, close to the transmitter or the receiver? On datasheet page 128, it suggest to place it close to the transmitter. But some other vendor suggest to place it close to the receiver, so I would like to double check with you which way is correct. Thank you.

  • 0
    TI__Mastermind 27905 points

    Hi Arie,

    We recommend placing capacitors closer to the transmitter, so whatever is driving the line, we would want to place the capacitors closest to that. You can see in that page in the datasheet that TX_M/TX_P capacitors are placed closest to MAC as the MAC is transmitting on those lines. While on RX_P/M, the PHY is transmitting on those lines. In those cases, we would want the capacitors closest to whatever is transmitting. Could you let me know what other vendor would want the capacitor closest to the receiver?

    Sincerely,

    Gerome

  • 0 in reply to Gerome Cacho
    TI__Expert 3390 points

    Hi Gerome,

    Thanks for your comment. The NAD module vendor recommend to put the capacitor close to the receiver, that's why I reach out to you to double confirm. Thanks.

  • 0 in reply to Arie Li
    TI__Mastermind 27905 points

    Hi Arie,

    Normally we do recommend having AC coupling caps near the transmitter as that has been seen in multiple designs. In this case of differing recommendations, we would side with the advise each device has on the line they are driving.

    Since the PHY is driving RX lines, follow the AC coupling recommendations for that. Since the NAD module vendor is driving the TX lines, follow the AC coupling recommendations there. 

    For this application, we would have AC coupling capacitors all near the PHY in this case.

    Sincerely,

    Gerome

  • 0 in reply to Gerome Cacho
    TI__Expert 3390 points

    Hi Gerome,

    Sorry I'm a little confused, so we need to follow the NAD vendor's suggestion, right? For the TX lines, when should we place the capacitor close to the transmitter, and when should we put it close to PHY? Thanks.

    Arie

  • 0 in reply to Arie Li
    TI__Mastermind 27905 points

    Hi Arie,

    It is a little unusual. Most applications would have the capacitors be near the transmitter as most vendors have that. The capacitors are more for the transmitting device rather than the receiving device on the lines. Most MAC vendors will say that for the TX lines to have it closer to MAC, as is convention. However, if this vendor says to place the capacitors near the PHY, then that is where they should go for this case only.

    Sincerely,

    Gerome